The present invention relates to a distance measuring apparatus used for cameras, video cameras, and the like for measuring the distance to an object, and more particularly to such an apparatus using an active method of distance measuring which employs CCD sensors as light receiving sensors.
One technique for the measurement of the distance to an object, the passive method, uses two light receiving sensors having the same optical system. These detect the contrast of an object in order to determine the distance in accordance with a shift in position caused by the parallax of the object as projected on each light receiving sensor. The passive method utilizes ambient light. Thus, it is possible to perform accurate distance measurement if the object is bright enough for the light receiving sensor to detect the object. However, the accuracy decreases when the object does not provide sufficient contrast or is dark.
Another technique for the measurement of the distance to an object, the active method, uses light emitted from a light emitting element towards the object, which is then reflected by the object and detected by system of light receiving elements. The position of the spectral center of the light reflected from the object is used to determine the distance by the triangulation method. With the active method, while distance measurement is possible when the object is dark, has no contrast, or has repeated patterns which reflect light, it is difficult to provide highly accurate range finding for an object located at a relatively large distance away due to a decrease in the quantity of light reflected from the object.
With operations using the active method, it has been found to be difficult to clearly distinguish whether the light reflected from the object is derived from ambient light or the light emitted by the light emitting element. Accordingly, it is difficult to detect the spectral center of the reflected light. Thus, the distance is not measurable and hence highly accurate distance measurement is not possible.
In some known distance measuring devices using the active method, a predetermined default distance value is utilized when the brightness of ambient light is greater than a predetermined level and a distance value is not measurable. However, this default distance value is set to a fixed value. Thus, even though the measurable distance may have changed due to variations in the brightness of the ambient light, the fixed default value is still utilized thereby making accurate distance measuring impossible.
Therefore, in known distance measuring apparatus using the active method, a light emitting element emitting infrared light has been used in combination with light receiving elements having a reduced sensitivity to visible light thereby making it possible to distinguish between the light emitted from the light emitting element and the ambient light.
However, it is then necessary to use a light emitting element and light receiving elements that are capable of emitting and receiving infrared light respectively. As a result, general purpose visible light emitting elements or light receiving elements can not be employed, and parts such as filters for separating infrared light from visible light are required. This makes it difficult to reduce the cost of the distance measuring apparatus using the active method.
If instead visible light is used for distance measuring using the active method, then the light receiving elements must determine the difference between the quantities of the light received in the case of light being reflected by an object with and without light emission by the light emitting element in order for the spectral center to be detected.
However, when the light receiving elements comprise CCD elements and in particular when they constitute the common light receiving elements for a distance measuring apparatus using both the passive and active methods, it is difficult to accurately detect the light emitted by the light emitting element simply by determining the difference between the quantities of light received by the CCD elements. Whilst a common integration time control can be applied, this can lead to saturation of the CCD elements making it impossible to detect the spectral center of the light emitted by the light emitting element and reflected from the object. Although light quantity control can be performed, this results in a constant maximum output level in both the case of light being emitted by the light emitting element and reflected from the object and in the case of just ambient light being reflected from the object without emission of light from the light emitting element. As a result, the spectral center of the light emitted by the light emitting element and reflected from the object can not be determined.